Method of making low frequency horn antenna



S pt, 5, 1967 E D. L. ANDERSON ETAL 3 39 275 METHOD OF MAKING LOW FREQUENCY HORN ANTENNA Filed April 15, 1964 2- Sheets-Sheet 1 INVENTORS DONALD L. ANDERSON KENNETH L. WALTON RICHARD E HUELSKAMP ATTORNEY SePt- 1.957 D. L. ANDERSON AL METHOD OF MAKING LOWF'REQUENCY HORN ANTENNA F i'led April 15, 1964 2 Sheets-Sheet 2.

PLACE AND IMPREG NATE LAMINAR SHEET ON PATTERN VACUUM BAG AND CU RE L ocATE FEED ELEMENT TUBE AND E DGE FLANGE REMOVE HALF HORN FROM PATTERN APPLY RELEASE AGENT, PEEL CLOTH AND FACING'SHEET REMOVE PEEL CLOTH AND APPLY METAL FILM SECURE HORN HALVES TOGETHER I. I. I. I I I.

w v v. N E O N M 080 TRT NEL EDA VI V N W H E W #N L R H DT A N H N N C v EI D K R. OJ Y B Q United States Patent Filed Apr. 15, 1964, Ser. No. 359,864 3 Claims. (Cl. 29-601) This invention relates to an improved technique for making and assembling a broadband ridged horn antenna,

The broadband automatic tracking system described in the copending application Ser. No. 406,106, filed Oct. 23, 1964, and assigned to the assignee of this application, includes a plurality of ridged waveguide horns capable of operating over a frequency range of 50 me. to 500 me. For operation in this frequency range, the horns are relatively large, having an overall length of 18 feet and an aperture diameter of 12 feet. The fabrication and assembly of such horns by the conventional process of electroforming each on a die is impracticably complex and prohibitively costly. Furthermore, an all-metal horn of this size is diificult to handle and transport, and the weight of an array of such horns, even when formed from lightweight metal such as aluminum, would impose severe loads on the bearings and drive mechanism of the tracking system and would limit the speed of response of the system.

An object of our invention is the provision of a method of making a broadband low frequency ridged horn antenna using lightweight non-metal materials in such a manner as to provide a structure having a greater strength to weight ratio than a corresponding metal structure.

Another object is the provision of a technique of assembling a low frequency horn of large dimensions that is compatible with the space limitations of modern transport vehicles and roadways.

A further object is the provision of the method of making a low frequency horn antenna of minimum weight and high strength for use in an automatic tracking system.

These objects are achieved in accordance with our invention by fabricating horn antennas with two substantially identical half sections, and securing these sections together at the site of erection of the antenna system in which the horns are used. Each half horn is formedon a permanent mold using synthetic fibrous sheet materials such as reinforced glass cloth. The sheet or cloth is impregnated with a resin, is vacuum sealed against the mold and is allowed to cure. The hardened half shell is then removed from the mold and the interior is coated with metal by flame spraying to provide the reflective or conductive surface for the horn. After shipment to the site, the pair of half sections are secured together along their edges to complete a horn antenna.

These and other objects of our invention will become apparent from the following description of a preferred embodiment thereof reference being had to the accompanying drawings in which:

FIGURE 1 is a side elevation, partly in section, of a ridged horn antenna made in accordance with our invention;

FIGURE 2 is a front view, partly in section, of the horn as viewed in line 22 of FIGURE 1;

FIGURE 3 is a rear perspective view of a permanent mold on which one-half of the horn antenna is formed;

FIGURE 4 is a flow diagram of the method of fabricating each half of the horn antenna;

FIGURE 5 is a greatly enlarged section of the horn wall on the mold;

FIGURES 6 and 7 are schematic outline drawings of the horn halves illustrating the manner in which they are connected together to form the complete antenna hornpand FIGURE 8 is a greatly enlarged transverse section of the connection of finished half horns to each other at their mating edges.

A ridged horn antenna made in accordance with this invention is shown in FIGURE 1 and comprises substantially identical half sections 10 and 11 secured together by flanges 12 which form the mating edges of the half sections. Since the horn half sections are identical except possibly for the lengths of inserted feed element tubes 22a and 22b, like reference characters indicate like parts on the drawings. The horn has a rectangular waveguide section 14 to which feed element 15 is connected on the horn center plane for transferring energy between the horn and external circuits. Waveguide section 14 is closed by a rear end Wall 16 and opens into a flared forward section 17 which preferably has a circular aperture 18 at its front end. Horn half sections 10 and 11 have longitudinally extending inwardly projecting preferably hollow ridges 20 which have a minimum inter-ridge spacing d (see FIGURE 2) toward the rear of the horn and which are forwardly longitudinally curved outwardly near the front end to a tapered junction with the wall of the horn at aperture 18.

Microwave feed element 15 is supported on the ridged waveguide section 14 and comprises a pair of conductive tubes 22a and 22b permanently integrated into the section at the rear of horn ridges 20, a center conductor 23 extending through tube 22a, and a cone 24 traversing the space between the ridges and nonconductively supported on tube 22b. Tube 22a and center conductor 23 connect to an external coupler 15a which projects from the waveguide for connection to appropriate transmitting or receiving circuits. The sprayed metal film 45 forming the conductive boundary wall on the interior of the horn makes electrical contact with tube 22a.

Each of the half sections 10 are formed on a mold 25 shown in FIGURE 3. Mold 25 comprises a hardened smooth-surfaced shell made of readily formable material such as plastic, or may be made of more durable material such as metal if needed for production of horns in large quantities. The mold has a shape conforming to that of the interior of each half horn and has a longitudinally extending ridge-defining recess 26 extending from the front of the mold to a step 27 at the rear thereof. A semi-cylindrical transverse recess 28 formed in the middle of the forward face of step 27 symmetrically of the central 1ongitudinal plane of the pattern serves to physically locate feed tube 22a or 22b in the proper position. A rim 30 extends outwardly from all sides of the bottom of the mold and provides a working surface for locating horn flange 12 and for the vacuum sealing steps as will be explained below.

The steps shown in block diagram in FIGURE 4 are followed in fabricating each half section of the horn with a composite wall shown in FIGURE 5. The outer surface of mold 25, including recess 26, initially is sealed if necessary, and is coated wtih a suitable release agent such as wax or polyvinyl chloride which lubricates the surface and facilitates separation of the half horn from the mold. A peel cloth 32 (see FIGURE 5) is laid against the entire working surface of the pattern and is impregnated with a thermosetting polyester resin. The peel cloth is a Fiberglas cloth designed to be stripped or peeled from the horn wall to provide a roughened or textured surface to which the sprayed metal film 45 more readilyadheres; the metal film is applied after the horn half section is lifted from the pattern. A facing sheet 33 made of glass cloth is next applied over the entire mold exterior against the peel cloth.

Conductive tube 22a or 22b is placed in recess 28 of rear step 27 so as to extend transversely of the pattern recess 26 from the bottom of the latter to the plane of the outer wall of the waveguide pattern part, and is supported in this position by a jig which extends over the side of the pattern. Tubes 22a and 22b may be identical in length or tube 22a may be longer so as to project outwardly from the waveguide for convenience in making external connections. Polyurethane foam 29 directed on step 27 around the tube serves to support the latter in position when the jig is later removed. Aluminum angle strips comprising the edge flange 12 (see FIGURE 8) are then placed on rim 30 tightly against the side and rear surfaces of the covered mold and the assembly is ready for the forming of the main body of the horn wall.

In accordance with our invention, the strength and rigidity of the horn antenna is derived primarily from the horn wall itself in conjunction with the longitudinal ridges 20 which not only enhance electrical performance of the antenna but also serve to reinforce and strengthen the structure. The mating flanges 12 also reinforce the horn. The form of horn wall we have found to provide the maximum strength to weight ratio for this antenna includes a unitary reinforced double-layer synthetic fibrous sheet 34 (see FIGURES and 8) as the main body of the wall. One type of such sheet material suitable for use in the horn wall is Raypan made by Raymond Manufacturing Company and consisting of two layers of interwoven Fiberglas cloth spaced by triangular-sectioned rigid foamed-plastic rods.

Sheet 34 is first applied to the mold recess 26 to form the body of ridge 20, and thereafter is applied to the outer mold surfaces to form the flared section 17 and waveguide section 14. The ridge wall is constructed with a strip 36 of sheet material 34, preferably one-half inch thick, placed along the bottom wall 26a of recess 26 from the front of the mold to step 27 and around conductive tube 22a or 22b. Strips 37a and 37b of the same one-half inch material are then cut to the shape and dimensions of sides 26b of recess 26 and are cemented to those sides for the entire length of the recess. Strips 36, 37a and 37b are impregnated with a polyester resin either before or after placement on the mold and are then pressed tightly against the walls 26a and 26b by the vacuum bag technique; i.e., a polyvinyl chloride film or sheet is spread over the materials on the mold and within recess 26, is sealed against rim 30 and is connected to a vacuum pump. After the bag is removed, lift plates 38, shown in broken line in FIGURES 1 and 2, are bonded to the side strips 37a and 37b and project out of the recess as shown. These plates provide strategically located anchor points within the horn shell for lifting it from the mold.

The remaining parts of the horn fall are constructed next. A cap or cover plate 40 (see FIGURE 2) of sandwich construction comprising facing sheet 33 (FIGURE 5), a reinforced body sheet 34, preferably one inch thick, and an outside facing sheet 41, is precut to overlay the open outer edges of the recess 26. Lift plates 38 extend up through suitable openings in plate 40 and an opening at the rear receives the upper end of tube 22a. The remainder of the mold is covered with a sheet 43 having the same sandwich-type construction to form the end wall 16 and the side wall of the Waveguide 14 and flared part 17, and is cemented to edge flange 12, see FIGURE 8. Resin is applied to the wall material before its placement on the pattern so that it is thoroughly impregnated. If a roughened texture on the outer surface of cap 40 and sheet 43 is desired to facilitate finishing, a peel cloth may be added.

The entire pattern is then covered with a vacuum bag which is sealed to rim 30 with an appropriate sealing compound and is connected to a vacuum pump. The wet Wall material is pressed tightly against the mold and the resin permeates the layers of the wall to insure adherence between these layers. The entire assembly is allowed to cure for 72 hours at a temperature of approximately 70 degrees Fahrenheit.

After the vacuum bag is removed, the shell constituting the half horn is separated from the mold by a hoist connected to lift' plates 38 and is inverted during the final steps of the process. The half horn is temporarily braced by transverse rods across its aperture 18, lift plates 38 are cut off at cap plate 40, the openings in the latter are sealed and all peel cloth is removed. The entire inside surface of the shell is then covered with a metal film 45, such as aluminum, which may be applied by flame spraying. This metal film is bonded to the inner end of tube 22a so as to make good electrical contact with the feed system. The metallized surface-is then painted with a suitable compound to seal and toughen the surface and to make it additionally weatherproof and resistant to chipping and cracking. Cone 24 of feed element 15 is mounted on an insulator on the inner end of tube 22b.

Two shells comprising half horns made in the manner described above are joined along the mating edges of flanges 12 (see FIGURES 6, 7 and 8) to form a complete horn, and the flanges are releasably secured together by bolts 46. The exterior of the horn may then be weatherproofed by painting. Center conductor 23 of feed element 15 is inserted through tube 22a into engagement with cone 24 and the horn is ready for operation. In practice, a ridged Waveguide horn of this type has a phase error which may be corrected by a suitable lens located in its aperture 18. A lens for this purpose is described in the copending application of Donald L. Anderson, Ser. No. 374,521, filed June 11, 1964, and assigned to the assignee of this application.

A ridged Waveguide horn fabricated in accordance with this invention and successfully tested in the tracking system described in the aforementioned patent application has the following dimensions and characteristics:

Horn length feet 18 Diameter of aperture 18 do 12 Wall thickness inches 1.06 Weight lbs 800 Approximate weight of aluminum horn having same dimensions lbs 1600 Flexural strength to weight ratio of described horn compared to corresponding aluminum horn 7.5/1.0

What is claimed is:

1. A method of making a horn antenna having opposed inwardly projecting spaced ridges extending longitudinally of the horn, consisting of the steps of applying a release agent to the surfaces of a mold having the shape of the interior of one-half of the horn antenna and having a bottom rim,

laying a peel cloth and a facing sheet over the mold surface,

mounting a conductive feed tube transversely of and at the rear of the longitudinal ridge-defining recess of the mold,

disposing a structural edge flange against the mold on the rim,

placing multilayer resin-impregnated fibrous sheets on the bottom and sides of the longitudinal recess, pressing said fibrous sheets against the mold and allowing same to cure,

cementing lift plates to the fibrous sheets defining the sides of the ridges,

placing similar multilayer resin-impregnated fibrous sheets over the remainder of the mold and cement- .ing same to the edge flange,

pressing said sheets against the mold and allowing same to cure whereby to form a half horn,

pulling on said lift plates and separating the half horn from the mold,

removing the lift plates and peel cloth,

flame spraying a metal film over the entire interior of the half horn including the inner edge of the conductive tube,

repeating the aforementioned steps to form a second half horn,

releasably joining the two opposed half horns at the edge flanges to form the complete horn, and

electrically connecting the conductive tubes to a micro wave feed assembly.

2. A method of making a horn antenna having opposed inwardly projecting spaced ridges extending longitudinally of the horn, consisting of the steps of applying a release agent to the surfaces of a mold having the shape of the interior of one-half of the horn antenna and having a bottom rim,

mounting a conductive feed tube transversely of and at the rear of the longitudinal ridge-defining recess of the mold,

disposing a structural edge flange against the mold on the bottom rim,

placing multilayer resin-impregnated fibrous sheets on the bottom and sides of the longitudinal recess,

pressing said fibrous sheets against the mold and allowing same to cure,

placing similar multilayer resin-impregnated fibrous sheets over the remainder of the mold and securing same to the edge flange,

pressing said sheets against the mold and allowing same to cure whereby to form a half horn,

separating the half horn from the mold,

flame spraying a metal film over the entire interior of the half horn including the inner edge of the conductive tube,

repeating the aforementioned steps to form a second half horn,

joining the two opposed half horns at the edge flanges to form the complete horn, and electrically connecting the conductive tubes to a microwave feed assembly. 3. A method of making a double-ridged horn antenna comprising two substantially identical half horns and utilizing a mold having the shape of the interior of one-half of the horn antenna and having a bottom rim, consisting of the steps of placing an edge flange against the mold on the bottom nm, placing multilayer resin-impregnated fibrous sheets against the mold and cementing same to the edge flange, pressing said sheets against the mold and allowing same to cure whereby to form a half horn, separating the half horn from the mold, applying a conductive film over the entire interior of the half horn, repeating the aforementioned steps to form a second horn half, joining the two opposed horn halves at the edge flanges to form the complete horn, and connecting a microwave feed element to the rear of the 5 horn.

References Cited UNITED STATES PATENTS 2,768,902 10/1956 Scholl 11711 XR 2,948,896 8/1960 Hart 343--91 2,998,922 9/1961 Gibson 117-105.2 XR 3,029,433 4/1962 Sokol 343-912 3,157,847 11/1964 Williams 33395 3,167,776 1/1965 Suliteanu 29- 155.5 XR

ROBERT F. WHITE, Primary Examiner.

T. J. CARVIS, Assistant Examiner, 

1. A METHOD OF MAKING A HORN ANTENNA OPPOSED INWARDLY PROJECTING SPACED RIDGES EXTENDING LONGITUDINALLY OF THE HORN, CONSISTING OF THE STEPS OF APPLYING A RELEASE AGENT TO THE SURFACES OF A MOLD HAVING THE SHAPE OF THE INTERIOR OF ONE-HALF OF THE HORN ANTENNA AND HAVING A BOTTOM RIM, LAYING A PEEL CLOTH AND A FACING SHEET OVER THE MOLD SUFACE, MOUNTING A CONDUCTIVE FEED TUBE TRANSVERSELY OF AND AT THE REAR OF THE LONGITUDINAL RIDGE-DEFINING RECESS OF THE MOLD, DISPOSING A STRUCTURAL EDGE FLANGE AGAINST THE MOLD ON THE RIM, PLACING MULTILAYER RESIN-IMPREGNATED FIBROUS SHEETS ON THE BOTTOM AND SIDES OF THE LONGITUDINAL RECESS, PRESSING SAID FIBROUS SHEETS AGAINST THE MOLD AND ALLOWING SAME TO CURE, CEMENTING LIFT PLATES TO THE FIBROUS SHEETS DEFINING THE SIDES OF THE RIDGES, PLACING SIMILAR MULTILAYER RESIN-IMPREGNATED FIBROUS SHEETS OVER THE REMAINER OF THE MOLD AND CEMENTING SAME TO THE EDGE FLANGE, PRESSING SAID SHEETS AGAINST THE MOLD AND ALLOWING SAME TO CURE WHEREBY TO FORM A HALF HORN, PULLING ON SAID LIFT PLATES AND SEPARATING THE HALF HORN FROM THE MOLD, REMOVING THE LIFT PLATES AND PEEL CLOTH, FLANGE SPRAYING A METAL FILM OVER THE ENTIRE INTERIOR OF THE HALF HORN INCLUDING THE INNER EDGE OF THE CONDUCTIVE TUBE, REPEATING THE AFOREMENTIONED STEPS TO FORM A SECOND HALF HORN, RELEASABLY JOINING THE TWO OPPOSED HALF HORNS AT THE EDGE FLANGES TO FORM THE COMPLETE HORN, AND ELECTRICALLY CONNECTING THE CONDUCTIVE TUBES TO A MICROWAVE FEED ASSEMBLY. 